1. Field of the Invention
The invention relates fibre gratings.
2. Description of Prior Art
The invention relates more particularly to tunable fibre lasers based on grating techniques which have applications for dense wavelength division multiplexing communication (DWDM) systems. A tunable fibre laser has a high pumping efficiency, is easy to fabricate and has a wide tuning range. However, difficulty of wavelength tuning accuracy and repeatability remain problems and are required for stringent requirements in lightwave communications, especially in DWDM systems.
Wavelength tuning of Bragg grating fibre lasers is easily achieved either thermally or mechanically. Direct thermal tuning however provides only a limited tuning range because of the low wavelength temperature coefficient and is difficult to control. Mechanical stretching or compressing is based presently on either piezoelectric transducers or stepping motors that inherently introduce non-linearity so tuning normally, and repeatability, are difficult to achieve.
It is an object of the invention to overcome at least reduce these problems.
According to the invention there is provided a tunable laser having a fibre Bragg grating anchored at each to relatively movable fixtures, an actuator coupled to the fixtures for creating relative movement that longitudinally stretches or compresses the fibre Bragg grating, measuring means arranged to directly monitor a longitudinal separation between the fixtures, and a microprocessor programmed to respond to signals from the measuring means and to control the actuator to change the separation between the fixtures to tune the laser to a desired wavelength.
The measurable means may be a linear variable differential transformer.
The measuring means may be a differential variable reluctance transformer.
A core of the transformer may be attached to one of the fixtures and a pick-up coil of the transformer attached to the other of the fixtures.
The measuring means may be an optical displacement sensor comprising twin optical fibres attached to one of the fixtures and a reflecting mirror attached to the other of the fixtures, the separation being monitored by a coupling loss between ends of the fibres and light reflected by the mirror